Kenworth T680 Overview
The T680 is a fuel-efficient aero cab for long-haul. Most run the PACCAR MX-13 with Eaton Fuller or PACCAR TX-12 automated transmission.
Top T680-Specific Issues
1. PACCAR MX-13 Aftertreatment
The MX-13 uses a two-box system (separate DOC/DPF and SCR canisters).
- SPN 3251 / FMI 0 — DPF soot critical
- SPN 3226 / FMI 3 — SCR efficiency below threshold
- SPN 5394 / FMI 17 — 7th injector (hydrocarbon dosing valve) performance issue
- SPN 3050 / FMI 0 — DPF outlet temp too high during regen
Key tip: The T680 has two NOx sensors (upstream and downstream of SCR). If downstream reads higher than upstream, the SCR catalyst is failing — replace the catalyst, not just sensors.
2. TX-12 Automated Transmission
- SPN 523 / FMI 14 — Transmission communication lost. Check harness connector at transmission top.
- Harsh or delayed shifts — Clutch actuator issue. Check actuator fluid level and clutch position sensor.
- Stuck in gear — Failed range cylinder or air supply issue.
3. SmartNav Display
- Screen freeze/reboot loop — Hard reset by holding power 10 seconds. May need dealer software update.
- Bluetooth drops — Known software issue, check firmware updates.
4. EGR Cooler Failure (MX-13)
The MX-13 EGR cooler is a known failure point:
- Symptoms: coolant loss with no visible leak, white exhaust smoke, coolant in charge air cooler piping
- SPN 411 / FMI 0 — EGR temperature high
- ⚠️ If you suspect EGR cooler failure, stop driving. Coolant entering the engine can cause hydrolock.
5. Cab and Chassis
- Wind noise from doors — Door seals shrink in cold weather. Replace secondary seal if whistling above 50 MPH.
- Ride height issues — Check air suspension bags and height control valve linkage.
Recommended Diagnostic Tools
- PACCAR ESA — Full dealer-level diagnostics for MX-13 and TX-12
- OTR Diagnostics — Good for code reading and forced regens
- Nexiq USB-Link 3 — Professional-grade J1939 interface